CN104601858B

CN104601858B - Image processing system and colour conversion method

Info

Publication number: CN104601858B
Application number: CN201410588740.1A
Authority: CN
Inventors: 宫永祐香
Original assignee: Kyocera Document Solutions Inc
Current assignee: Kyocera Document Solutions Inc
Priority date: 2013-10-31
Filing date: 2014-10-28
Publication date: 2018-01-02
Anticipated expiration: 2034-10-28
Also published as: US9232113B2; CN104601858A; JP5940039B2; EP2869548A1; US20150116740A1; JP2015088968A; EP2869548B1

Abstract

The present invention relates to image processing system and colour conversion method.The device includes interpolation reference data generating unit, reference space coordinate calculating part, interpolation arithmetic unit and color conversion portion.Interpolation reference data generating unit is in the reference space after the first color space is divided into multiple form and aspect faces, form and aspect face is split by grid point, chroma of the generation in form and aspect face with lower part and is higher than defined threshold part in defined threshold, changes by the interpolation reference data after the shape of the triangle formed for the grid point of linear interpolation.Reference space coordinate calculating part calculates coordinate of the colour of the first color space in reference space with reference to interpolation reference data.The coordinate for the triangle that interpolation arithmetic unit is made up of the grid point in the form and aspect face adjacent by the coordinate with calculating carries out linear interpolation, generates color transformation table.Color conversion portion is converted to the colour of the first color space the colour of the second color space by color transformation table.High-precision color transformation table is generated according to the present invention.

Description

Image forming apparatus and color value conversion method

Technical Field

The present invention relates to an image forming apparatus and a color value conversion method for generating a high-precision color conversion table.

Background

There are image forming apparatuses such as a multi-function Peripheral (MFP) capable of printing documents and images. In the normal print data of the image forming apparatus, color values of RGB (Red, Green, Blue) color space are used. On the other hand, in outputting to recording paper by the image forming apparatus, color values in CMYK (Cyan, Magenta, Yellow, black or Key plane) color spaces are often used. Therefore, the image forming apparatus needs to convert color values of the RGB color space into color values of the CMYK color space to perform image formation.

There is a simple conversion formula for color conversion from an RGB color space to a CMYK color space:

C＝(1－R－K)/(1－K)

M＝(1－G－K)/(1－K)

Y＝(1－B－K)/(1－K)

K＝min(1－R，1－G，1－B)

however, performing color conversion with good accuracy using the above equation is difficult in a CMYK color space depending on a model. Therefore, a color conversion table (LUT) is used for color conversion from the RGB color space to the CMYK color space. As such a color conversion table, when RGB is 8 bits each, the data amount is excessive if CMYK of output values or the assignment of color spaces independent of devices (CIE XYZ color space, CIE LAB/CIE LCh color space, etc.) is maintained for input values of about 1678 ten thousand colors which are the third power of 256 gradations of each color. Therefore, a color conversion table that reduces the amount of data in an intermittent manner instead of the full color of the RGB color space is generally generated and stored in the image forming apparatus.

In such a color conversion table with a reduced data amount, assignment from RGB color values to CMYK color values is set for each lattice point obtained by dividing an RGB color space into predetermined intervals. For color values other than the positions of the lattice points, color values of CMYK are calculated by interpolation operation. In the interpolation operation, a linear interpolation, an interpolation based on an approximate expression, or the like is used. For example, an image processing apparatus is known which registers information for obtaining upper bits from lattice point information and lower bits from an interpolation ratio in advance, and performs N-point interpolation using the information.

Disclosure of Invention

An image forming apparatus of the present invention includes: an interpolation reference data generation unit, a reference spatial coordinate calculation unit, an interpolation calculation unit, and a color conversion unit. The interpolation reference data generation unit generates interpolation reference data by dividing a RGB color space into a plurality of color planes by grid points in the reference space, the interpolation reference data being obtained by changing the shape of a triangle formed by the grid points for linear interpolation between a portion where the chroma of the color plane is equal to or less than a predetermined threshold value and a portion higher than the predetermined threshold value. The reference space coordinate calculation unit calculates the coordinates of the color values of the RGB color space in the reference space with reference to the interpolation reference data generated by the interpolation reference data generation unit. The interpolation calculation unit performs linear interpolation using the coordinates of a triangle formed by the lattice points in the color tone plane adjacent to the coordinates calculated by the reference space coordinate calculation unit, and generates a color conversion table indicating a correspondence relationship between color values in the RGB color space and color values in the CMYK color space. The color conversion section converts the color values of the RGB color space into color values of the CMYK color space by the color conversion table. Wherein,

the reference space is calculated by associating color values of the RGB color space, which are color value data of interpolated point data registered as the color conversion table, with color values of the CIE LAB color space,

the predetermined threshold value represents a threshold value of CIE C which is referred to when the shape of the triangle set as the triangle data is changed by the color value of the CIE LAB color space calculated from the color value of the RGB color space of each lattice point data of the hue plane data,

the interpolation reference data generation unit includes:

a lattice point color value calculation section that calculates a color value of the RGB color space in the lattice point in the CIELAB color space; and

a triangle shape information specifying unit that specifies information on a shape of a triangle formed by the lattice points, based on the color values in the CIE LAB color space calculated by the lattice point color value calculation unit,

the triangle shape information determination unit performs a triangle data change process to change one vertex of the triangle if a lattice point at a position obtained after +1 or-1 exists in the directions of the CIE C component and the CIE L component when the CIE C component is a lattice point of the lattice point data equal to or less than the predetermined threshold value, and changes the triangle if the lattice point exists in the direction of the position obtained after +1 or-1 in the directions of the CIE C component and the CIE L component in correspondence with the changed vertex for the triangle forming a pair,

the interpolation arithmetic unit includes:

an adjacent triangle calculation unit that calculates coordinates of the triangle formed by the lattice points in each of two adjacent color planes of the RGB color space corresponding to each interpolated point, and sets the calculated coordinates of the triangle as tetrahedral data;

a tetrahedron dividing unit configured to divide a triangular prism formed by coordinates of the triangle formed by the lattice points in two adjacent color planes into tetrahedrons with reference to the tetrahedron data;

a tetrahedron inside/outside determination section that determines whether or not the interpolated point of the color conversion table exists inside for each of the tetrahedrons divided by the tetrahedron division section; and

a volume interpolation unit that calculates an interpolation value of a color value of the interpolated point in the CIE LAB color space based on a volume ratio of a solid formed by the lattice point of the tetrahedron and the interpolated point when the tetrahedron interior/exterior determination unit determines that the interpolated point is located inside the tetrahedron,

the adjacent triangle calculating unit (1) performs triangle search processing when the interpolated point is located on the hue plane, calculates the triangle in which the interpolated point is inside, linearly interpolates and calculates the color values in the CIE LAB color space by the distance from each vertex of the triangle in the reference space, calculates the color values in the CMYK color space corresponding to the calculated color values in the CIE LAB color space with reference to the setting data of the color conversion table, and sets the color values as the color value data of the corresponding interpolated point data; (2) when the interpolated points are surrounded by two adjacent hue surfaces, triangle calculation processing is carried out, two vertical lines are respectively led out from the hue surfaces, the intersection point of the vertical lines is calculated to be positioned in the triangle, the triangle pillar surrounded by the triangle of each of the hue surfaces is formed, and the tetrahedron data is set as

The color value conversion method of the present invention includes: an interpolation reference data generation step of generating interpolation reference data in which a triangle shape formed by the lattice points for performing linear interpolation is changed between a portion where a chroma of the hue plane is equal to or less than a predetermined threshold value and a portion where the chroma is higher than the predetermined threshold value, by dividing the hue plane by the lattice points in a reference space in which an RGB color space is divided into a plurality of hue planes; a reference space coordinate calculation step of calculating coordinates of color values of the RGB color space in the reference space with reference to the generated interpolation reference data; an interpolation calculation step of performing linear interpolation from the coordinates of a triangle formed by the lattice points in the hue plane adjacent to the calculated coordinates to generate a color conversion table indicating a correspondence relationship between color values in the RGB color space and color values in the CMYK color space; and a color conversion step of converting the color values of the RGB color space into color values of the CMYK color space by the color conversion table. Wherein,

the interpolation reference data generating step includes:

a lattice point color value calculation step of calculating color values of the RGB color space in the lattice points in the CIELAB color space; and

a triangle shape information determination step of determining information on the shape of a triangle formed by the lattice points from the calculated color values in the CIE LAB color space,

the triangle shape information specifying step includes a triangle data changing step of changing the triangle data when the CIE C component is a lattice point of the lattice point data equal to or less than the predetermined threshold value,

the triangle data change processing step includes: a step of changing one vertex of the triangle at a lattice point of a position obtained after +1 or-1 exists in the directions of the CIE C component and the CIE L component; and a step of changing the triangle to be a pair if the lattice point exists in a direction of a position obtained after +1 or-1 in the direction of the CIE C component and the CIE L component corresponding to the changed vertex,

the interpolation operation step includes:

an adjacent triangle calculation step of calculating coordinates of the triangle formed by the lattice points in each of two adjacent color planes of the RGB color space color values corresponding to the interpolated points, and setting the calculated coordinates of the triangle as tetrahedral data;

a tetrahedron division step of dividing a triangular prism formed by coordinates of the triangle formed by the lattice points in two adjacent color phase planes into tetrahedrons with reference to the tetrahedron data;

a tetrahedral interior/exterior determination step of determining whether or not the interpolated point of the color conversion table exists inside for each of the divided tetrahedrons; and

a volume interpolation step of calculating an interpolation value of a color value of the interpolated point in the CIELAB color space based on a volume ratio of a solid formed by a lattice point of the tetrahedron and the interpolated point when the interpolated point is determined to be located inside the tetrahedron,

the adjacent triangle calculating step has a triangle retrieval processing step when the interpolated point is located on the hue plane, the triangle retrieval processing step including: calculating the triangle in which the interpolated point is an inner one; a step of performing linear interpolation and calculation on the color values of the CIE LAB color space by the distance on the reference space from each vertex of the triangle; and a step of calculating color values of the CMYK color space corresponding to the calculated color values of the CIE LAB color space with reference to setting data of the color conversion table, and setting the color values as color value data of corresponding interpolated point data,

the adjacent triangle calculating step has a triangle calculation processing step when the interpolated point is surrounded by two adjacent hue surfaces, the triangle calculation processing step including: drawing vertical lines from the two hue planes, respectively, and calculating the triangle with the intersection point of the vertical lines positioned inside; and forming the triangular prism surrounded by the triangles of the two color phase planes, and setting the triangular prism as the tetrahedron data.

Drawings

Fig. 1 is a diagram showing a block configuration of a control unit and a storage unit of an image forming apparatus according to an embodiment of the present invention.

Fig. 2 is a diagram illustrating a flow of interpolation reference data generation processing according to an embodiment.

Fig. 3 is a diagram conceptually showing a color conversion table setting reading process according to an embodiment.

Fig. 4 is a diagram conceptually showing the hue plane data generation processing according to the embodiment.

Fig. 5 is a diagram illustrating a flow of interpolation processing of interpolated points according to the embodiment of the present invention.

Fig. 6 is a diagram conceptually showing the adjacent hue plane calculation processing according to the embodiment.

Fig. 7 is a diagram illustrating a flow of a detailed process of the interpolation arithmetic processing according to the embodiment.

Fig. 8 is a diagram conceptually showing the tetrahedron segmentation processing according to the embodiment.

Fig. 9 conceptually illustrates a tetrahedron segmentation process according to an embodiment.

Fig. 10 is a diagram showing a flow of color conversion rendering processing according to an embodiment.

Fig. 11 conceptually shows the triangular shape of the color surface in comparative example 1 of the present invention.

Fig. 12 is a graph showing the interpolation result of comparative example 1.

Fig. 13 is a diagram conceptually showing the triangular shapes of the hue surfaces in comparative examples 2a and 2b of the present invention.

Fig. 14 is a graph of interpolation results of comparative examples 2a and 2 b.

Fig. 15 is a diagram conceptually showing the triangular shape of the color phase plane of the embodiment of the present invention.

Fig. 16 is a graph of interpolation results of the embodiment.

Fig. 17 is a diagram showing a functional block configuration of an image forming apparatus according to an embodiment of the present invention.

Fig. 18 is a diagram showing an external and internal configuration of an image forming apparatus according to an embodiment.

Detailed Description

Structure of image forming apparatus

First, the configuration of the image forming apparatus 1 will be described with reference to fig. 17. The image processing portion 11, the document reading portion 12, the document feeding portion 13, the conveying portions (the paper feed roller 42b, the conveying roller pair 44, and the delivery roller pair 45), the network transceiver 15, the operation panel portion 16, the image forming portion 17, the FAX transceiver 18, and the storage portion 19 of the image forming apparatus 1 are connected to the control portion 10 via a common bus or the like. Each unit is controlled by the control unit 10.

The control Unit 10 is an information Processing Unit such as a General Purpose Processor (GPP), a Central Processing Unit (CPU), a Microprocessor (MPU), a Digital Signal Processor (DSP), a Graphic Processing Unit (GPU), and an Application Specific Processor (ASIC). The control unit 10 reads out a control program stored in the ROM or HDD of the storage unit 19, expands the control program in the RAM, and executes the control program. Accordingly, the control unit 10 operates as each unit of the functional blocks described later. The control unit 10 may control the entire apparatus in accordance with predetermined instruction information input from an external terminal or an operation panel unit 16, not shown.

The image Processing Unit 11 is a control arithmetic Unit such as a Digital Signal Processor (DSP) or a Graphics Processing Unit (GPU). The image processing unit 11 is a unit that performs predetermined image processing on image data. For example, the image processing unit 11 performs various image processing such as enlargement and reduction, density adjustment, gradation adjustment, and image improvement. The image processing unit 11 stores the image read by the document reading unit 12 in the storage unit 19 as print data 500. At this time, the image processing unit 11 can also convert the print data 500 into a file unit in a format such as PDF or TIFF.

The document reading unit 12 is a unit that reads (scans) a set document. The document feeding section 13 is a unit that conveys a document read by the document reading section 12. The image forming unit 17 is a unit that forms an image on a recording sheet based on data stored in the storage unit 19 and read by the document reading unit 12 or acquired from an external terminal, in accordance with an output instruction from a user. The transport unit transports recording paper from a paper feed cassette 42a (fig. 18), forms an image on the recording paper by the image forming unit 17, and then transports the recording paper to the stack tray 50. The operations of the document reading unit 12, the document feeding unit 13, the conveying unit, and the image forming unit 17 will be described later.

The network transmitter/receiver 15 is a network connection unit including a LAN card, a wireless transmitter/receiver, and the like for connecting to an external network such as a LAN, a wireless LAN, a WAN, or a mobile telephone network. The network transceiver 15 transmits and receives data via a data communication line and transmits and receives audio signals via a voice telephone line.

The operation panel unit 16 includes: a display unit such as an LCD; a numeric keypad, a start key, a cancel key, a switch button for switching operation modes such as copy, FAX transmission, scan, and the like; and an input unit such as a button and a touch panel for executing an instruction relating to execution of a job related to printing or transmission or saving or recording of a selected document. The operation panel unit 16 acquires instructions of various jobs of the image forming apparatus 1 from a user. Further, the information of each user may be input and/or changed in accordance with the instruction of the user acquired from the operation panel unit 16.

The FAX transmitting/receiving unit 18 is a unit for performing transmission/reception of facsimile, and is connected to a normal telephone line, an ISDN line, or the like. Further, FAX transmitting/receiving unit 18 stores the received facsimile image in storage unit 19 as print data 500 (fig. 1). In addition, FAX transmission/reception unit 18 may perform facsimile transmission instead of recording the drawing data stored in storage unit 19 by image forming unit 17.

The storage unit 19 is a storage unit using a semiconductor Memory such as a Read Only Memory (ROM) or a Random Access Memory (RAM), or a recording medium such as a Hard Disk Drive (HDD). The RAM of the storage unit 19 retains the stored contents by a function such as self-refresh even in the power-saving state. A control program for controlling the operation of the image forming apparatus 1 is stored in the ROM or HDD of the storage unit 19. In addition, the storage unit 19 stores account settings of the user. Further, the storage unit 19 may include an area for saving folders for each user.

In the image forming apparatus 1, the control unit 10 and the image processing unit 11 may be integrally formed in a manner of a CPU or the like built in a GPU or a chip module package. The control unit 10 and the image processing unit 11 may have a RAM, a ROM, a flash memory, and the like.

Operation of image Forming apparatus

Next, the external and internal configurations of the image forming apparatus 1 according to an embodiment of the present invention and their operations will be described with reference to fig. 18. The document reading unit 12 is disposed above the main body 14. The document feeding section 13 is disposed above the document reading section 12. The stack tray 50 is disposed on the side of the output port 41 formed in the main body 14 for outputting the recording paper. The operation panel unit 16 is disposed on the front side of the image forming apparatus 1.

The document reading section 12 includes: a scanner 12a, a platen glass 12b, and an original reading slit 12 c. The scanner 12a is composed of an exposure lamp, a Charge Coupled Device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS) imaging sensor, and the like. The scanner 12a is configured to be movable in a document conveying direction of the document feeding section 13. The platen glass 12b is a document table made of a transparent member such as glass. The original reading slit 12c has a slit formed in a direction orthogonal to the original conveying direction of the original feeding portion 13.

When reading an original placed on the platen glass 12b, the scanner 12a is moved to a position opposed to the platen glass 12 b. The scanner 12a scans a document placed on the platen glass 12b, reads the document to acquire image data, and outputs the acquired image data to the control unit 10 provided in the main body 14 (fig. 17). When reading the original conveyed by the original feeding portion 13, the scanner 12a is moved to a position opposed to the original reading slit 12 c. The scanner 12a reads the original document in synchronization with the original document conveying operation of the original document supply portion 13 via the original document reading slit 12c to acquire image data, and outputs the acquired image data to the control portion 10 provided in the main body portion 14.

The document feeding section 13 includes: a document placing section 13a, a document output section 13b, and a document conveying mechanism 13 c. The original placed on the original placement portion 13a is sequentially fed out one by the original conveying mechanism 13c, conveyed to a position facing the original reading slit 12c, and then output to the original output portion 13 b. The document feeding section 13 is configured to be reversible. By raising the document feeding portion 13 upward, the upper surface of the platen glass 12b can be opened.

The main body 14 includes an image forming portion 17, and includes a paper feeding portion 42, a paper conveying path 43, a conveying roller pair 44, and an output roller pair 45. The paper feed unit 42 includes a plurality of paper feed cassettes 42a and a plurality of paper feed rollers 42 b. The plurality of paper feed cassettes 42a store recording paper sheets having different sizes or orientations. The paper feed roller 42b feeds out recording paper sheets one by one from the paper feed cassette 42a to the paper conveyance path 43. The paper feed roller 42b, the conveyance roller pair 44, and the delivery roller pair 45 function as a conveyance unit. The recording paper is transported by the transport unit. The recording paper guided to the paper conveyance path 43 by the paper feed roller 42b is conveyed to the image forming unit 17 by the conveyance roller pair 44. Then, the recording paper on which recording has been performed by the image forming unit 17 is discharged to the stack tray 50 by the discharge roller pair 45.

The image forming section 17 includes: a photosensitive drum 17a, an exposure portion 17b, a developing portion 17c, a transfer portion 17d, and a fixing portion 17 e. The exposure section 17b is an optical unit including a laser device, a mirror, a lens, an LED array, and the like. The exposure portion 17b outputs laser light or the like in accordance with image data, and exposes the photosensitive drum 17a, thereby forming an electrostatic latent image on the surface of the photosensitive drum 17 a. The developing portion 17c is a developing unit that develops the electrostatic latent image formed on the photosensitive drum 17a with toner. The developing portion 17c forms a toner image based on the electrostatic latent image on the photosensitive drum 17 a. The transfer section 17d transfers the toner image formed on the photosensitive drum 17a by the developing section 17c onto recording paper. The fixing unit 17e heats the recording paper to which the toner image is transferred by the transfer unit 17d, and fixes the toner image to the recording paper.

Structure of control unit and storage unit of image forming apparatus 1

Referring to fig. 1, the configuration of the control unit 10 and the storage unit 19 of the image forming apparatus 1 will be described. The image forming apparatus 1 includes: an interpolation reference data generation unit 100, a reference spatial coordinate calculation unit 110, an interpolation calculation unit 120, and a color conversion unit 130. The storage unit 19 stores interpolation reference data 200, threshold value data 210, adjacent hue plane data 220, color conversion table 230, tetrahedron data 240, and print data 500.

The interpolation reference data generation unit 100 divides the hue plane by the grid points in a reference space in which the RGB color space is divided into a plurality of hue planes, and generates "interpolation reference data 200 in which the shape of a triangle formed by grid points for performing linear interpolation is changed in a portion where the chroma of the hue plane is equal to or less than a predetermined threshold value and a portion higher than the predetermined threshold value". At this time, the interpolation reference data generation unit 100 sets the lattice points as the lattice point data 300 of the hue plane data 201 of the interpolation reference data 200, and changes the shape of the triangle formed by the lattice points as the triangle data 310.

The reference space coordinate calculation unit 110 refers to the interpolation reference data 200 generated by the interpolation reference data generation unit 100, and calculates coordinates in a base (reference) space used in the interpolation reference data 200 for the color values of the RGB color space corresponding to each interpolated point.

The interpolation arithmetic unit 120 generates the color conversion table 230 by performing linear interpolation using the coordinates of a triangle formed by lattice points in the color phase plane adjacent to the coordinates calculated by the reference spatial coordinate calculation unit 110.

The color conversion section 130 converts the color values of the RGB color space into color values of the CMYK color space based on the generated color conversion table 230.

The interpolation reference data generation unit 100 includes: a lattice dot color value calculation unit 101 and a triangle shape information specification unit 102. The cell point color value calculation unit 101 calculates color values in the CIE LAB color space (hereinafter, the color space based on the expression of CIE LAB/CIE LCh is simply referred to as "CIE LAB color space") of the color values in the RGB color space in the cell points. The triangle shape information specifying unit 102 specifies and/or changes the shape of a triangle formed by lattice points from the color values in the CIE LAB color space of the lattice point data 300 of the hue plane data 201 calculated by the lattice point color value calculation unit 101. The triangle shape information specifying unit 102 sets the specified and/or modified triangle shape as the triangle data 310.

The reference space coordinate calculation unit 110 includes: an adjacent hue plane calculation unit 111. The adjacent hue plane calculation section 111 calculates a hue plane adjacent to the color value of the RGB color space corresponding to each interpolated point. At this time, the adjacent hue plane calculation unit 111 searches the hue plane data 201 for each interpolated point included in the interpolated point data 232 of the color conversion table 230 as to which hue plane of the interpolation reference data 200 the interpolated point is located on or between the two hue planes? The adjacent hue plane calculation unit 111 sets the information of the searched hue plane as the adjacent hue plane data 220.

The interpolation arithmetic unit 120 includes: an adjacent triangle calculation unit 121, a tetrahedron division unit 122, a tetrahedron inside/outside determination unit 123, and a volume interpolation unit 124. The adjacent triangle calculating unit 121 refers to the adjacent hue surface data 220, and calculates the coordinates of a triangle formed by lattice points on each of two hue surfaces adjacent to the color value of the RGB color space corresponding to each interpolated point calculated by the adjacent hue surface calculating unit 111. The adjacent triangle calculating unit 121 sets the calculated coordinates of the triangle as the tetrahedron data 240. The tetrahedron segmentation unit 122 segments the triangular prism into tetrahedrons by referring to the tetrahedron data 240. The triangular prism is formed by the coordinates of a triangle formed by lattice points in two adjacent color planes calculated by the adjacent triangle calculating unit 121. The tetrahedron segmentation unit 122 sets coordinates of the segmented triangular prism and the like as tetrahedron data 240. The tetrahedron inside/outside determination unit 123 refers to the tetrahedron data 240, and determines whether or not the interpolated point of the color conversion table 230 is present inside each of the tetrahedrons divided by the tetrahedron division unit 122. When the tetrahedron inside/outside determination unit 123 determines that the interpolated point is located inside the tetrahedron, the volume interpolation unit 124 calculates the volume ratio of the solid formed by the lattice points of the tetrahedron and the interpolated point with reference to the tetrahedron data 240. The volume interpolation unit 124 calculates an interpolation value of color values of the interpolated point in the CIE LAB color space from the calculated volume ratio.

The interpolation reference data 200 is data of a reference space into which the RGB color space is divided into a plurality of color planes, which is referred to for generating the color conversion table 230.

The threshold value data 210 is data indicating a threshold value of CIE C referred to when changing the shape of a triangle set as the triangle data 310 by a color value in the CIE LAB color space calculated from the RGB color values of the lattice point data 300 of the hue plane data 201. The threshold data 210 may be set by a device-dependent value such as color rendering in the CMYK color space of the image forming apparatus 1, for example.

The adjacent hue plane data 220 is data indicating a hue plane of the reference space adjacent to the interpolated point of the interpolated point data 232 retrieved from the color conversion table 230. For example, when the interpolated point of the interpolated point data 232 is located on the hue plane of the reference space, the adjacent hue plane data 220 is set as the hue plane data 201 indicating the hue plane. When the interpolated point is not located on the hue plane, the adjacent hue plane data 220 includes a value indicating two adjacent hue planes.

The color conversion table 230 includes values converted into a CMYK color space in which each vertex of a cube divided at a predetermined interval in an RGB color space is set as an interpolated point, setting values for division, and the like.

The tetrahedron data 240 is data representing a structure of a tetrahedron obtained by dividing a triangular prism formed by triangles of two hue surfaces adjacent to each other, the triangles being "interpolated points held by the adjacent hue surface data 220". The tetrahedron data 240 includes data indicating the coordinates and volume ratio of two tetrahedrons connecting the triangles of the two color planes and the interpolated point, respectively.

The print data 500 may be image data read by the document reading section 12, image data received by the FAX transmission/reception section 18, data of a Page Description Language (PDL) acquired from an external terminal or the like via the network transmission/reception section 15 or an external recording medium, data of an electronic document Format such as a Portable Document Format (PDF), or image data such as JPEG (jpg) or GIF or bitmap. The print data 500 may also contain various setting data. The print data 500 may also include attribute information such as a file name, a file type, a generation time, an update time, and a viewing time. The print data 500 may include data of an object unit for drawing characters, images, and the like.

The interpolation reference data 200 includes hue plane data 201. The hue plane data 201 is data including the coordinates, structure, color values, and the like of the hue plane into which the reference space is divided. The hue plane data 201 includes, for example, each hue plane divided into 6 or more by taking the achromatic color axis of the RGB color space as the center and the directions of the vertexes of the RMBCGY as the hues. The hue plane in the hue plane data 201 may have a triangular shape in which each of K and W, which are the ends of achromatic colors, is connected to a point of a peak (hereinafter, referred to as a peak point), which is the maximum chroma value in the RGB color space. The hue plane data 201 may also include the values of the coordinate system of CIE LCh.

The hue plane data 201 includes lattice point data 300 and triangle data 310. The lattice point data 300 is data such as coordinates and color values of a lattice point obtained by dividing the hue plane into a plurality of pieces. As the lattice point data 300, for example, the color plane is divided by the division number corresponding to the division number of each gradation of the color conversion table 230. Further, data as color values of the lattice point data 300 may also include color values in an RGB color space, color values in a CIE LAB color space, and color values in a CMYK color space. Further, the color values in the CIE LAB color space of the lattice point data 300 may also include coordinate values after being converted into the coordinate system of the CIE LCh. The triangle data 310 is structural data of the lattice point data 300 referred to for linear interpolation. The triangle data 310 may also be shown by a pointer or the like of the lattice point data 300.

Further, the color conversion table 230 includes setting data 231 and interpolated point data 232. The setting data 231 includes the number of gradation levels of the RGB color space of the color conversion table 230, the number of divisions into a cube, the setting of each vertex of the divided cube as an interpolated point, the setting of the number of gradation levels of the CMYK color space, a predetermined device-dependent value of the image forming apparatus 1, a calibrated device-dependent value, and the like. Color value data 400 for assigning colors of the RGB color space to colors of the CMYK color space in the interpolated points set at equal intervals is set in the interpolated point data 232. The color value data 400 of the interpolated point data 232 may have a data format such as a structure including the gray scale data of C, M, Y, K colors. The color value data 400 may include, as an intermediate value, a color value in the CIE LAB color space corresponding to a color value in the RGB color space and a color value in the CMYK color space corresponding to the CIE LAB color space by gamut mapping. When the color conversion table 230 is built in the product after calculation, only the color value data 400 for assigning colors in the CMYK color space may be provided as the color conversion table 230.

Here, the control unit 10 of the image forming apparatus 1 functions as an interpolation reference data generation unit 100, a reference spatial coordinate calculation unit 110, an interpolation calculation unit 120, and a color conversion unit 130. Each part of the image forming apparatus 1 described above is a hardware resource for executing each process of the color conversion table generating method and the color conversion table generating program according to the present invention.

Interpolation reference data generation processing of image forming apparatus

Next, an interpolation reference data generation process of the image forming apparatus 1 according to the embodiment of the present invention will be described with reference to fig. 2 to 4E. The interpolation reference data generation processing according to the present embodiment generates hue plane data 201 including a hue plane obtained by dividing an RGB color space into a plurality of parts, for example. The interpolation reference data generation unit 100 calculates coordinates of grid points as grid point data 300 for each color plane of the generated color plane data 201. The interpolation reference data generation unit 100 also generates triangle data 310 for linear interpolation from the calculated lattice point data 300. At this time, the interpolation reference data generating unit 100 changes the shape of the triangle data 310 when the value of CIE C is equal to or less than a predetermined threshold value in the value obtained by changing the RGB color values of the lattice point data 300 to the CIE LAB color space. The interpolation reference data generation processing of the present embodiment is mainly performed by the interpolation reference data generation unit 100 of the control unit 10 cooperating with each unit and executing a program stored in the storage unit 19 using hardware resources. Next, the interpolation reference data generation processing will be described in detail for each step with reference to fig. 2.

Step S101

First, the control unit 10 performs a color conversion table setting reading process. The control unit 10 reads the size, division number, and the like of the RGB color space used in the reference space for interpolation from the setting data 231 of the color conversion table 230.

The concept of the interpolated points of the color conversion table 230 will be described with reference to fig. 3 (a) and 3 (b). Fig. 3 (a) shows an example in which an RGB color space, which is an additive color space, is expressed as a cube in a three-dimensional space. That is, the cube of fig. 3 (a) shows the positions of the respective colors when the colors are added in the directions of the R (red), G (green), and B (blue) axes from K (black). For example, when the gradations of the respective RGB colors are expressed by 8 bits (0 to 255), black, that is, K is (0, 0, 0), R is (255, 0, 0), G is (0, 255, 0), and B is (0, 0, 255), may be expressed as the vertices of the cube. The vertex of a color obtained by adding two colors of RGB is expressed as Y (yellow) RGB (255, 255, 0), C (cyan) RGB (0, 255, 255), and M (magenta) RGB (255, 0, 255). W (white), which is a vertex obtained by adding all of the three RGB colors, is expressed as (255, 255, 255). Fig. 3 (b) conceptually shows a color conversion table 230 that generates a color conversion table 230 in which points obtained by dividing each element of the RGB color space at equal intervals are interpolated points. As shown in the above example, when RGB is 8-bit gradation, it is not practical to store all the color assignments of CMYK color spaces having a cubic volume of about one thousand to seven million of 256(8 bits). Therefore, the gradation is divided, and the CMYK color assignment is stored for each interpolated point orthogonal in the RGB color space. In the example of fig. 3 (b), an example is shown in which interpolated point data 232 of the color conversion table 230 is generated in which 8-bit gradation is divided into four, the number of lattices in each axis is set to 5, and the CMYK colors are assigned to each interpolated point. In the color conversion table 230, the color values in the CMYK color space of the RGB colors excluding the interpolated points are calculated by the color conversion process described later using the color values in the CMYK color space of the neighboring interpolated points.

Step S102

Next, the control unit 10 performs a hue plane data generation process. The control unit 10 divides the hue of the reference space into a predetermined number to generate hue plane data 201. The generation of the hue plane data 201 will be described with reference to conceptual diagrams of fig. 4 (a) and 4 (b).

The control unit 10 first calculates a relationship between the color value of the RGB color space and the color value of the CIE LAB color space at each interpolated point. That is, the color values of the RGB color space of the color value data 400 of the interpolated point data 232 registered as the color conversion table 230 are made to correspond to the color values of the CIE LAB. Accordingly, a reference space for interpolation is calculated. According to fig. 4 (a), the print data 500 to be input includes color values in the RGB color space. However, as described above, the image forming apparatus 1 outputs color values of the CMYK color space. In the present embodiment, the input RGB color space is converted into the CIE LAB color space, which is a device-independent color space. Then, the relationship between the color values in the CMYK color space of the image forming apparatus 1 on the output side and the color values in the CIE LAB color space, which is a device-independent color space, is calculated. By performing color gamut mapping on the input CIELAB color values to the output CIE LAB color values, the color conversion table 230 for converting the input RGB color space color values to the output CYMK color space color values can be calculated. In addition, an RGB color space corresponding to an input profile, such as sRGB or Adobe RGB, may be used for input. That is, the RGB color space corresponding to the input profile may be temporarily converted into color values in the CIE LAB color space, and the color values in the CIE LAB color space may be converted into color values in the CMYK color space.

The conversion from the orthogonal coordinates of the RGB color space used in the color conversion table 230 to the coordinate system of the reference space using the hue plane will be described with reference to fig. 4 (b) and 4 (c). Fig. 4 (b) shows an example in which the relationship with the CIE LAB color space is calculated by using, as a basic color, a color that becomes a vertex in the orthogonal coordinates of the RGB color space, which is the same cubic space as fig. 3 (a). A line connecting K (black) and W (white) in the RGB color space becomes a gray axis of achromatic color. That is, achromatic colors of RGB (0, 0, 0), RGB (1, 1, 1), RGB (2, 2, 2) to RGB (255, 255, 255) are located on the axis from K to W. On the other hand, the position and chroma in the CIE LAB color space can be expressed by a gray axis having a line segment connecting K and W as a main axis and hues obtained by connecting RMBCGY (red, magenta, blue, cyan, green, yellow). In addition, the dots on the line segment obtained by connecting the RMBCGY of the RGB color space are dots with the maximum chroma (vividness). Fig. 4 (c) shows a color plane obtained by dividing the RGB color space by a predetermined angle around the gray axis connecting K and W and calculating a point on a line segment having the maximum chroma (vividness). The calculated coordinates of the hue plane and the like are set as hue plane data 201. Fig. 4 (d) is a front view of the RGB color space with the gray axis erected and seen from above. Here, for one of the calculated hue plane data 201, a point at which chroma (vividness) is the largest is shown as P0. Fig. 4 (e) shows an example in which one hue plane of the hue plane data 201 shown in fig. 4 (d) is selected and expressed in the coordinate system of the CIE LAB color space. A triangle obtained by connecting the gray axis connecting K and W to the point P0 having the highest chroma (vividness) is a color plane.

Step S103

Here, the control unit 10 performs the lattice point calculation processing. The control unit 10 divides the hue plane of the calculated hue plane data 201, and calculates a lattice point serving as a basis of a triangle for linearly interpolating an interpolation point. That is, the control unit 10 calculates coordinates of grid points spaced at predetermined intervals for each color plane, and sets the coordinates as the grid point data 300. The control unit 10 may calculate, for each of the lattice points, coordinates corresponding to the CIE L component and the CIE C component on each color plane in addition to the coordinates in the RGB color space by the lattice point color value calculation unit 101. The coordinates corresponding to the CIE L component, CIE C component correspond to color values of the CIE LAB color space. That is, the control unit 10 may convert the coordinate space of the orthogonal system into a coordinate system for each color plane in the reference space. This makes it possible to simplify the search during linear interpolation. Fig. 4 (e) shows an example in which the control unit 10 calculates a grid point on the hue plane of the hue plane data 201. The vertex of each triangle is represented by W, K and the point P0 as lattice points.

Step S104

the control unit 10 calculates a triangle (triangle surface) formed by three lattice points for interpolation for each color plane by the triangle shape information specifying unit 102, and sets the triangle data α. in this embodiment, a combination of specific triangles is specified because the interpolation is performed by a triangle obtained by connecting the lattice points of the color planes of the color plane data 201. in fig. 4 (e), three adjacent lattice points represented by black dots of the lattice point data 300 are set as the triangle data α. in this step, the control unit 10 generates the triangle data α in which the lattice points are simply connected to form a triangle shape, and the triangle data α may be a triangle in which a line segment passing through each lattice point in a line segment parallel to a line segment connecting W to P0 passes through each point, a line segment passing through each lattice point in a line segment parallel to a line segment connecting K to P0 passes through each point, and a line segment passing through each lattice point in a line segment parallel to an axis connecting K to W, or a triangle in which a line segment passing through each lattice point in a line segment parallel to an axis connecting K to W passes through β, and a triangle data β is a triangle in a triangle shape of a triangle of which is formed by a triangle of a triangle, or a triangle of a triangle, and a triangle of a.

Step S105

Next, the control unit 10 determines whether or not the CIE C component is equal to or less than a threshold value with respect to the lattice point data 300 of each color plane data 201. The control unit 10 determines that lattice points exist in a low chroma portion as lattice points for which the CIE C component of the lattice point data 300 is equal to or less than the threshold value, and determines yes. The control unit 10 determines that the grid points having the CIE C component larger than the threshold value are "no" as the grid points existing in the high chroma portion. For the yes grid point, the control unit 10 advances the process to step S106 and executes it. For the no lattice point, the control unit 10 advances the process to step S107.

Step S106

When the CIE C component is the lattice point of the lattice point data 300 equal to or less than the threshold value, the control unit 10 performs the triangle data change process. The control unit 10 changes the shape of the triangle data 310 by the triangle shape information specifying unit 102. The control unit 10 changes one vertex of the triangle so that the grid point is changed if there is a grid point at a position obtained after +1 or-1 in the direction of the CIE C component and the CIE L component, for example. At this time, the control unit 10 changes the pair of triangles if the lattice points are present in the direction of the position obtained after +1 or-1 in the directions of the CIE C component and CIE L component, corresponding to the changed vertices. That is, the shape of the modified triangle is, for example, the shape γ or the shape δ shown in fig. 13 and 15. Further, a process may be performed in which a triangle having a shape different between the low saturation portion and the high saturation portion is prepared from the beginning at each coordinate position of all the grid points.

Step S107

Here, the control unit 10 determines whether or not the triangle data 310 of all the color surface data 201 is generated. The control unit 10 determines yes when the triangle data 310 of all the color surface data 201 is generated and the shape change is completed. Otherwise, the control unit 10 determines "no". If "yes", the control unit 10 considers that the generation of the interpolation reference data 200 is completed, and ends the interpolation reference data generation processing. If "no", the control unit 10 returns the process to step S103, and continues to generate the triangle data 310 from the lattice point data 300 of the remaining color plane data 201. As described above, the interpolation reference data generation processing according to the embodiment of the present invention is ended.

Interpolation processing of interpolated points in image forming apparatus

Next, the processing of the image forming apparatus 1 according to the embodiment of the present invention will be described with reference to fig. 5 to 9 (B). In the interpolation process of the interpolated point according to the present embodiment, in order to facilitate the search in the reference space, the coordinate system of the interpolated point from the RGB color space to the CIE LAB color space is converted. Then, from the calculated coordinates of the interpolated point, a search is made as to which hue plane of the hue plane data 201 of the interpolated reference data 200 the interpolated point surrounds. Then, an interpolation operation is performed by the grid points of the searched color surface. The interpolation processing of the interpolated point according to the present embodiment is mainly performed by the reference spatial coordinate calculation unit 110 and the interpolation calculation unit 120 of the control unit 10 in cooperation with each unit, and by executing a program stored in the storage unit 19 using hardware resources. Next, with reference to fig. 5, an outline of interpolation processing of interpolated points will be described for each step.

Step S201

First, the control unit 10 performs interpolated point acquisition processing by the reference spatial coordinate calculation unit 110. The control unit 10 reads interpolated point data 232 for performing an interpolation operation based on linear interpolation from the color conversion table 230.

Step S202

Next, the control unit 10 performs interpolation point coordinate calculation processing by the reference space coordinate calculation unit 110. The control unit 10 calculates the coordinates in the reference space of the interpolation reference data 200 from the coordinates in the RGB color space of the interpolated point data 232. This makes it possible to facilitate the search for the hue plane and the triangle in the reference space.

Step S203

Next, the control unit 10 performs the adjacent hue plane calculation process by the adjacent hue plane calculation unit 111 of the reference spatial coordinate calculation unit 110. The control unit 10 calculates adjacent color planes using the coordinates of the interpolation point in the reference space. That is, the control unit 10 searches for which hue plane the interpolated point exists on or between two hue planes from the hue plane data 201 of the interpolation reference data 200 based on the coordinates of the interpolated point in the interpolated point data 232? The control unit 10 sets the hue plane of the searched hue plane data as the adjacent hue plane data 220. By using the coordinates described above, it is possible to easily determine which color plane the interpolated point is located on or surrounded by.

The relationship between the interpolation point Q and the adjacent color phase plane in the RGB color space when performing interpolation calculation using lattice points will be described with reference to the conceptual diagrams of fig. 6 (a) and 6 (b). Fig. 6 (a) shows an example in which the interpolation point Q is sandwiched between the hue plane of the two hue plane data 201-1 and the hue plane of the hue plane data 201-2. Since the interpolation points obtained by dividing the RGB color space by a cube may not be on the same plane as the hue plane, two hue planes are searched for in this way. Fig. 6 (b) shows that the interpolation point Q exists in a space sandwiched between a triangle surrounded by three lattice points on the hue plane of the hue plane data 201-1 and a triangle surrounded by three lattice points on the hue plane of the hue plane data 201-2.

Step S204

Next, the control unit 10 performs interpolation operation processing by the interpolation operation unit 120. The control unit 10 reads the adjacent hue surface data 220 in which the searched hue surface is set, and interpolates the interpolated points using the lattice points of the hue surface. When the interpolated point is located on the hue plane of the hue plane data 201, the control unit 10 performs interpolation using a triangle of a lattice point of the hue plane. When the interpolated point is sandwiched between the two hue planes of the hue plane data 201, the control unit 10 generates a solid from the lattice points of the hue plane and performs interpolation calculation. Details of this processing will be described later. This concludes the interpolation process of the interpolated point according to the embodiment of the present invention.

Here, the details of the interpolation arithmetic processing in the interpolated point interpolation processing will be described with reference to fig. 7 and conceptual diagrams of fig. 8 (a) to 9 (b).

Step S301

Next, the control unit 10 determines whether or not the interpolated point is determined to be on the hue plane by the adjacent triangle calculation unit 121 of the interpolation calculation unit 120. The control unit 10 determines yes when the interpolated point is on the color plane. In addition, the control unit 10 determines no in the other cases, that is, when the interpolated point is surrounded by two hue planes. If "yes", the control unit 10 advances the process to step S302. If no, the control unit 10 advances the process to step S303.

Step S302

When the interpolated point is located on the color plane, the control unit 10 performs the triangle search process by the adjacent triangle calculation unit 121 of the interpolation calculation unit 120. The control unit 10 calculates a triangle in which the interpolated point is located inside, and linearly interpolates the color values in the CIE LAB color space by the distance in the reference space from each vertex of the triangle to calculate the color values. The control unit 10 calculates color values in the device-dependent CMYK color space corresponding to the calculated color values in the CIE LAB color space with reference to the setting data 231 in the color conversion table 230, and sets the color values as the color value data 400 of the corresponding interpolated point data 232. Then, the control unit 10 ends the interpolation arithmetic processing.

Step S303

When the interpolated point is surrounded by two adjacent color planes, the control unit 10 performs the triangle calculation process by the adjacent triangle calculation unit 121 of the interpolation computation unit 120. The control unit 10 draws a perpendicular line to each of the two color planes, and calculates a triangle in which the intersection of the perpendicular lines is located inside. The control unit 10 forms a triangular prism surrounded by the respective triangles of the two color planes, and sets the triangular prism as the tetrahedron data 240.

Step S304

Next, the control unit 10 performs a tetrahedron division process by the tetrahedron division unit 122 of the interpolation operation unit 120. The control unit 10 divides a triangular prism surrounded by the respective triangles of the two color picture surfaces into three tetrahedrons. As described with reference to fig. 8 (a) and 8 (b), and fig. 9 (a) and 9 (b), the triangular prism obtained by these two triangles is generally divided into three tetrahedrons. Fig. 8 (a) shows an example of a triangular prism surrounded by two triangles. The triangular prism is formed by a triangle connecting lattice points a1, a2, and A3 of the hue plane data 201-1 and a triangle connecting lattice points B1, B2, and B3 of the hue plane data 201-2. Fig. 8 (b) shows a shape obtained when the triangular prism in fig. 8 (a) is divided into three tetrahedrons. In fig. 8 (b), four points are selected from triangular columns, and three tetrahedrons are formed. However, when two triangles are in contact with the gray axis, a1 indicates the same position as B1, a2 indicates the same position as B2, and the volume of a tetrahedron other than the tetrahedron connecting lattice points B1-a2-A3-B3 is 0, and thus it cannot be used in interpolation calculation. Therefore, interpolation operation is performed using tetrahedrons other than them. Fig. 9 (a) shows another example of a triangular prism surrounded by two triangles. The triangular prism is formed by a triangle formed by lattice points a1, a2, and A3 of the hue plane data 201-1 and a triangle formed by lattice points B1, B2, and B3 of the hue plane data 201-2. Fig. 9 (b) shows a shape obtained when the triangular prism in fig. 9 (a) is divided into three tetrahedrons. In fig. 9 (b), four points are also selected from the triangular columns, and three tetrahedrons are formed. When the gray axis is connected, a2 indicates the same position as B2, and therefore the volume of the tetrahedron connecting lattice points B1-a2-B3-B2 is 0, and this is not used in the interpolation operation. At this time, interpolation operation is also performed using tetrahedrons other than the tetrahedrons.

Step S305

Here, the control unit 10 performs the inside/outside determination process by the tetrahedral inside/outside determination unit 123 of the interpolation computation unit 120. The control unit 10 performs, for each of the three tetrahedrons formed, inside-outside determination for comparing the coordinates to calculate whether or not the interpolated point exists inside.

Step S306

Next, the control unit 10 determines whether or not the interpolated point is inside the tetrahedron by the tetrahedron inside/outside determination unit 123 of the interpolation computation unit 120. The control unit 10 determines yes when the interpolated point is inside any one of the three tetrahedrons as a result of the above-described inside-outside determination. In addition, the control unit 10 determines no when the interpolation point is not located inside the tetrahedron. If "yes", the control unit 10 advances the process to step S307. If no, the control unit 10 advances the process to step S308.

Step S307

When the interpolated point is inside the tetrahedron, the control unit 10 performs the volume rate interpolation value calculation process by the volume interpolation unit 124 of the interpolation computation unit 120. When the control unit 10 determines that the interpolated point is located inside in the above-described inside-outside determination, it calculates the color value of the interpolated point in the CIE LAB color space from the lattice points using the volume ratio. At this time, the control unit 10 calculates a ratio of a volume of a solid formed by a lattice point of each vertex of the tetrahedron and an interpolated point, performs linear interpolation based on the ratio, and calculates a color value in the CIE LAB color space. The control unit 10 calculates color values of the device-dependent CMYK color space corresponding to the color values of the CIE LAB color space with reference to the setting data 231 of the color conversion table 230, and sets the color values as the color value data 400 of the corresponding interpolated point data 232. Then, the control unit 10 ends the interpolation arithmetic processing.

Step S308

When the interpolated point is not located inside the tetrahedron, the control unit 10 determines whether or not the interpolated point is not located inside all of the three tetrahedrons by the tetrahedron inside/outside determination unit 123 of the interpolation computation unit 120. The control unit 10 determines yes when the interpolated point is not inside any of the three tetrahedrons. In addition, the control unit 10 determines no in the other cases, that is, when it is determined that all of the three tetrahedrons are not outside. If "yes", the control unit 10 advances the process to step S309. If "no", the control unit 10 returns the process to step S305, and continues to determine whether the interpolated point is inside any other of the three tetrahedrons.

Step S309

When the interpolated points are not located inside all of the three tetrahedrons, the control unit 10 performs other triangular prism selection processing by the adjacent triangle calculation unit 121 of the interpolation computation unit 120. The control unit 10 selects a candidate of a triangular prism from other lattice points around the interpolated point in two adjacent color planes. Then, the control unit 10 returns the process to step S301, and proceeds to the process. This concludes the interpolation arithmetic processing according to the embodiment of the present invention.

Color conversion rendering process of image forming apparatus

Next, a color conversion drawing process of the image forming apparatus 1 according to the embodiment of the present invention will be described with reference to fig. 10. The color conversion rendering process of the present embodiment performs color conversion using the generated color conversion table 230 for the print data 500, and outputs the color converted data. The color conversion drawing process of the present embodiment is mainly performed by the color conversion unit 130 of the control unit 10 cooperating with each unit and executing a program stored in the storage unit 19 using hardware resources. Next, the color conversion drawing process will be described in detail for each step with reference to the flowchart of fig. 10.

Step S401

First, the control section 10 performs print data acquisition processing. The control section 10 reads an original set in the original reading section 12 and acquires it as print data 500. In this case, the object contained in the print data 500 is mainly image data of each page. Further, the control section 10 may acquire the print data 500 facsimile-received from the FAX transmission/reception section 18. In this case, the objects included in the print data 500 are also mainly image data of each page. The control unit 10 may also acquire the print data 500 transmitted from another terminal or a server (not shown) from the network transmission/reception unit 15. In this case, the print data 500 may include text data and image data for each page. Further, the control section 10 may also acquire the print data 500 from a recording medium (not shown) connected to the outside. In this case, the print data 500 may include text data and image data for each page. The control unit 10 stores the acquired print data 500 in the storage unit 19.

Step S402

Next, the control section 10 performs color conversion processing. The control section 10 performs interpolation calculation on the color values in the RGB color space of each object included in the print data 500 by using the color value data 400 of the interpolated point corresponding to the interpolated point data 232 of the color conversion table 230, and converts the color values into color values in the CMYK color space.

Step S403

Next, the control unit 10 performs output processing. The control section 10 draws the print data 500 with the converted color values and outputs it through the image forming section 17. Accordingly, the page in the state in which the color values of the RGB color space of the print data 500 are color-converted into the color values of the CMYK color space is output. The control unit 10 may output the synthesized drawing data by generating a PDF or PS file, recording the file on an external recording medium, transmitting the file to an external terminal or a server, transmitting the file by facsimile, or the like. In this way, the color conversion rendering process according to the embodiment of the present invention is ended.

With the above configuration, the following effects can be obtained. Conventionally, in the technique of patent document 1, although a solid body obtained by lattice points referred to for interpolation used for gamut mapping can be freely changed, there is a possibility that an interpolation value may be largely changed due to the solid body shape, and the accuracy of the color conversion table 230 that can be generated is low. In contrast, the image forming apparatus 1 according to the embodiment of the present invention is an image forming apparatus that generates a color conversion table 230 for indicating a correspondence relationship between color values in a first color space and color values in a second color space, and includes: an interpolation reference data generation unit (100) for dividing a hue plane by a grid point in a reference space in which a first color space is divided into a plurality of hue planes, generating interpolation reference data (200) in which the saturation of the hue plane is equal to or less than a predetermined threshold value and is higher than the predetermined threshold value, and changing the shape of a triangle formed by grid points for performing linear interpolation; a reference space coordinate calculation unit 110 that calculates coordinates of the color value of the first color space in the reference space with reference to the interpolation reference data 200 generated by the interpolation reference data generation unit 100; and an interpolation calculation unit 120 that generates a color conversion table 230 by performing linear interpolation using the coordinates of a triangle formed by lattice points in the color phase plane adjacent to the coordinates calculated by the reference space coordinate calculation unit 110. With this configuration, it is possible to improve the interpolation accuracy when interpolating the interpolated point with respect to the interpolation reference data 200 in the reference space by linear interpolation. Therefore, the color conversion table 230 with high accuracy can be generated. Further, by using the color conversion table 230, it is possible to perform high-precision color conversion processing without requiring complicated calculation and holding enormous data.

In the image forming apparatus 1 according to the embodiment of the present invention, the first color space is an RGB color space, the second color space is a CMYK color space, and the interpolation reference data generating unit 100 includes: a lattice point color value calculation unit 101 that calculates a color value in a CIE LAB color space of a color value of an RGB color space in a lattice point; and a triangle shape information specifying unit 102 that specifies information on the shape of a triangle formed by lattice points from the color values in the CIE LAB color space calculated by the lattice point color value calculation unit 101. The interpolation arithmetic unit 120 is characterized by comprising: a tetrahedron dividing unit 122 for dividing a triangular prism formed by coordinates of a triangle formed by lattice points in two adjacent color planes into tetrahedrons; a tetrahedron inside/outside determination unit 123 for determining whether or not the interpolated point of the color conversion table 230 exists inside each tetrahedron divided by the tetrahedron division unit 122; and a volume interpolation unit 124 that calculates an interpolation value of a color value of the interpolated point in the CIE LAB color space based on a volume ratio of a solid formed by the lattice point of the tetrahedron and the interpolated point when the tetrahedron inside/outside determination unit 123 determines that the interpolated point is located inside the tetrahedron. With this configuration, the color conversion table 230 that can convert from the RGB color space to the CMYK color space with high accuracy can be generated. In addition, when performing color conversion, a desired color can be obtained by using the CIE LAB color space instead of the XYZ color space. Further, since the method of selecting the interpolated point of the color conversion table 230 used for the interpolation is devised, it is possible to output a color value corresponding to an assumed value and to improve the accuracy of the color conversion.

Further, an image forming apparatus 1 according to an embodiment of the present invention includes: the color conversion unit 130 converts the color values in the first color space into color values in the second color space by using the generated color conversion table 230. With this configuration, it is possible to provide an image forming apparatus with improved color conversion accuracy by using the high-accuracy color conversion table 230. Further, since complicated calculation is not required, the cost of calculation for color conversion can be reduced.

Other embodiments

Further, the present invention can also be applied to a color conversion apparatus other than the image forming apparatus. That is, a network scanner, a server separately connected to a scanner via USB, or the like may be used. Further, a general-purpose computer such as a PC or a server may be used as the image forming apparatus. The color conversion table generated by the image forming apparatus can be provided in a computer for converting a moving image or an apparatus for assigning colors to a display. In the above-described embodiment, the RGB color space and the CMYK color space are described as the color spaces, but other color spaces may be used. For example, a configuration may be adopted in which color conversion is performed for a color space to which colors other than CMYK are added. With this configuration, a highly accurate color conversion table conforming to human vision can be generated and used.

In addition, although the present embodiment shows an example in which one color conversion table 230 is used, color conversion may be performed using two color conversion tables of a front stage part and a rear stage part. In this case, the interpolation reference data 200 can be used in the color conversion table 230 of the front stage section that performs gamut mapping from the RGB color space to the device-independent color space (CIE LAB/CIE LCh, etc.). The color values after being converted into the device-independent color space by the color conversion table 230 of the front stage section are converted into the device-dependent color space (CMYK, etc.) by the color conversion table 230 of the rear stage section and output. With this configuration, even if the color conversion device deteriorates with time or the color value changes due to replacement or calibration of the device, high-precision color conversion can be performed simply by separately preparing the color conversion table of the subsequent stage section. Further, a process of converting the color values converted into the CIE LAB color space into the CMYK color space in the subsequent stage portion may be performed by performing color conversion of the print data from the preceding stage portion using another color conversion table provided in the OS of the external PC or the like. In addition, the color conversion table 230 in the front stage can be easily readjusted by changing the predetermined threshold value of the CIE C component.

In the above-described embodiment, the interpolated points are not limited to the equal intervals, and the intervals between the interpolated points may be set at unequal intervals based on design values of color expression of the image forming apparatus 1, measurement values of human color recognition, and the like. In addition, a detailed color conversion table may be prepared separately in the vicinity of the gray axis. Thus, the print data can be output by performing more accurate color conversion suitable for human vision.

The present invention will be further described below by way of comparative examples and examples based on the drawings, but the following specific examples do not limit the present invention.

Comparative example 1

first, a description will be given of comparative example 1 in which linear interpolation is performed on a shape such as a set of triangles simply divided by a line segment passing through each grid point, by using (a) of fig. 11 to 12 and (b) of fig. 12, fig. 11 is a conceptual diagram showing a hue plane of hue plane data 202 of comparative example 1 including 45 grid points each divided into eight parts in the horizontal and vertical directions and a triangle used for interpolation, in comparative example 1, a triangle simply using a shape α and a shape β obtained by inverting the shape α in the left and right directions, fig. 12 (a) is an example in which a hue plane and its grid points are expressed in an XYZ coordinate system of CIE C and CIE L in an interpolation reference space, fig. 12 (a) is an example in which a hue plane of comparative example 1 of fig. 11 is actually assigned to a CIE LAB color space by using a coordinate system of CIE LCh, fig. 12 (b) is an example in which the size and shape of each triangle of the hue plane are different, and fig. 12 (b) is an example in which linear interpolation color values are calculated by using a CIE space where linear interpolation of a middle point of each triangle, and a linear interpolation color value of a CIE space is expressed by using a CIE C, and a linear interpolation is not expressed by a linear interpolation color value of a linear interpolation partial interpolation of a CIE cross-bar chart showing a CIE color point.

Comparative examples 2a and 2b

fig. 13 is a conceptual diagram showing hue surface data 203 obtained by changing the shape of a triangle when the triangle which is a unit of interpolation is changed for the entire hue surface, as in comparative examples 2a and 2b, 45 lattice points are provided as in comparative example 1, and fig. 13 is a conceptual diagram showing hue surface data 203 obtained by changing the shape of a triangle when the hue surface is changed, comparative examples 2a and 2b use the shape γ and the shape δ for the entire hue surface, and use the shape α only at the end portions of comparative example 2a where the shape γ and the shape δ cannot be changed and the chroma is high, fig. 14 (a) shows the shape of a triangle in fig. 13, and when the enlarged view is similarly to comparative example 1, it is known that interpolation is not smoothly performed as in comparative example 1, and fig. 14 (b) shows the shape of a triangle in fig. 13, but does not perform linear interpolation using the color space of a LAB, and the variation of the interpolated color is not significantly reduced as in the enlarged view of the CIE C, and the CIE C is known that the variation is not significantly reduced.

Examples

fig. 15 to 16 are conceptual diagrams of hue surface data 201 of the example, in the example, the same triangle shapes as in comparative examples 2a and 2b are used for 45 lattice points similar to comparative examples 1 and 2a and 2b except for the end portions of the portions where CIE C is lower than the predetermined threshold, in the hue surface, the shapes γ and δ are used for the portions where CIE C is lower than the predetermined threshold except for the end portions, and the interpolation calculation is performed using the shapes α and β for the portions where CIE C is higher than the threshold, in the hue surface, fig. 16 shows the shape of fig. 15, and in the same manner as in comparative example 2b, in the case where the intermediate points of the respective triangles are linearly interpolated in the CIE LAB color space, it is known that the deviation of the interpolation results is reduced for the portions where CIE C is higher and the portions where CIE C is lower, and the interpolation results are smoothed.

Claims

1. An image forming apparatus for converting color values by a color conversion table,

the image forming apparatus includes:

an interpolation reference data generating unit that generates interpolation reference data by dividing a RGB color space into a plurality of color planes by grid points in a reference space, the color planes being divided by grid points, the interpolation reference data being obtained by changing a shape of a triangle formed by the grid points for performing linear interpolation between a portion where a chroma of the color plane is equal to or less than a predetermined threshold value and a portion where the chroma of the color plane is higher than the predetermined threshold value;

a reference space coordinate calculation unit that calculates coordinates of the color values of the RGB color space in the reference space with reference to the interpolation reference data generated by the interpolation reference data generation unit;

an interpolation calculation unit that generates a color conversion table indicating a correspondence relationship between color values in the RGB color space and color values in the CMYK color space by performing linear interpolation using the coordinates of a triangle formed by the lattice points in the hue plane adjacent to the coordinates calculated by the reference space coordinate calculation unit; and

a color conversion section converting color values of the RGB color space into color values of the CMYK color space by the color conversion table,

the interpolation reference data generation unit includes:

a lattice point color value calculation unit that calculates color values of the RGB color space in the lattice points in the CIE LAB color space; and

the interpolation arithmetic unit includes:

the adjacent triangle calculating unit, (1) when the interpolated point is located on the hue plane, performing triangle search processing to calculate the triangle in which the interpolated point is inside, linearly interpolating color values in the CIE LAB color space by a distance in the reference space from each vertex of the triangle to calculate, referring to setting data of the color conversion table, calculating color values in the CMYK color space corresponding to the calculated color values in the CIE LAB color space, and setting the color values as color value data of the corresponding interpolated point data; (2) when the interpolated point is surrounded by two adjacent hue surfaces, triangle calculation processing is carried out, a vertical line is respectively led out from the two hue surfaces, the intersection point of the vertical line is calculated to be positioned in the triangle, the triangle pillar surrounded by the triangle of each of the two hue surfaces is formed, and the tetrahedron data is set.

2. A color value conversion method for converting a color value by a color conversion table,

the color value conversion method includes:

an interpolation reference data generation step of generating interpolation reference data in which a triangle shape formed by the lattice points for performing linear interpolation is changed between a portion where a chroma of the hue plane is equal to or less than a predetermined threshold value and a portion where the chroma is higher than the predetermined threshold value, by dividing the hue plane by the lattice points in a reference space in which an RGB color space is divided into a plurality of hue planes;

a reference space coordinate calculation step of calculating coordinates of color values of the RGB color space in the reference space with reference to the generated interpolation reference data;

an interpolation calculation step of performing linear interpolation from the coordinates of a triangle formed by the lattice points in the hue plane adjacent to the calculated coordinates to generate a color conversion table indicating a correspondence relationship between color values in the RGB color space and color values in the CMYK color space; and

a color conversion step of converting the color values of the RGB color space into color values of the CMYK color space by the color conversion table,

the interpolation reference data generating step includes:

a lattice point color value calculation step of calculating color values of the RGB color space in the lattice points in the CIE LAB color space; and

the interpolation operation step includes:

a volume interpolation step of calculating an interpolation value of a color value of the interpolated point in the CIE LAB color space based on a volume ratio of a solid formed by a lattice point of the tetrahedron and the interpolated point when the interpolated point is determined to be located inside the tetrahedron,